The present invention relates to a triboelectric spray gun for the electrostatic spraying of powder paint.
The principle of a triboelectric spray gun for the electrostatic spraying of powder paint and the following.
The paint, in insulating powder form, conveyed by “transport” air, is electrostatically charged by friction against an insulating material, capable of withdrawing electrons therefrom, so that the powder becomes positively charged.
As is generally known, a triboelectric spray gun operating according to this principle comprises:                a powder intake,        an air intake,        a duct transporting the air and the powder, while ensuring that the powder becomes electrically charged; and        in front of said charging duct means for emitting at least one jet of air/powder mixture.        
The insulating material, ensuring that the powder becomes electrostatically charged by friction, forms the wall of the aforementioned duct that transports the air and powder. This insulating material is electrically connected to ground via joints between components of the spray gun, so that it remains overall electrically neutral, its charge permanently flowing away by “creep” (flow over the surface), to ground. Of course, the higher the friction force between the powder and the walls of the charging duct, the greater or better the electrical charging of the powder.
Many documents have already described this kind of triboelectric spray gun. As examples, mention may be made here of European Patent Application EP 0 163 118, EP 0 199 054, EP 0 314 049, EP 0 592 137 and EP 0 818 245, or else International Patent Applications WO 88/08332, WO 92/11950 and WO 98/25707.
Most of the triboelectric spray guns described in those documents have, as charging ducts, a relatively elongate annular space bounded by parts against which the powder transported by the air will rub and be charged. The annular charging duct is bounded internally by a central core that advantageously has, in profile, a wavy shape (see in particular the aforementioned documents EP 0 314 049 (FIG. 3), EP 0 592 127 and WO 92/11950) so as to increase the stirring of the powder, and therefore to facilitate its contact with the insulating material provided for charging it. The central core is held in place inside an external body, by which the charging duct is bounded externally, by centering elements mounted at each end of said duct. This central core may have a conical end on the side on which the inlet of the charging duct lies, so as to distribute the powder in the annular space forming this duct; the other end of the central core, located on the side on which the outlet of the charging duct lies, may also be of conical shape, so as to collect the charged powder before it is sent into small outlet channels belonging to or associated with the means for emitting jets of air/powder mixture, these means being in practice formed by a nozzle.
As is usual, in the rear part of the triboelectric spray gun, an air intake is formed along the central axis of this spray gun. However, in most known constructions, an additional intake for air, and if necessary for powder, is placed so as to be orthogonal to the central axis of the spray gun, upstream of the entry cone of the charging duct—see for example the aforementioned documents EP 0 592 137 and EP 0 818 245. The presence of an obstacle-forming “orthogonal” injection means creates perturbations; in particular, it is the cause of premature wear and/or of powder building up at the stagnation point.
Moreover, in current triboelectric spray guns, the grounding, especially of the central core of the charging duct, requires the addition of parts that complicate the set-up; this grounding is generally made around the external periphery and toward the rear of the body of the spray gun. Owing to the electrical forces and fields employed by the charges created, it is frequently observed that arcing occurs between the inner portion of the spray gun and its grounding region, and this leads to parts that are not, per se, wear parts having to be replaced relatively frequently.